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From: Peter Zijlstra <peterz@infradead.org>
To: Peter Oskolkov <posk@google.com>
Cc: Jann Horn <jannh@google.com>, Peter Oskolkov <posk@posk.io>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
	Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
	Andrei Vagin <avagin@google.com>,
	Thierry Delisle <tdelisle@uwaterloo.ca>
Subject: Re: [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers
Date: Tue, 14 Sep 2021 20:15:30 +0200	[thread overview]
Message-ID: <YUDmwsf8Lf7HQ3+s@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YUDkR8YtekMkuaBH@hirez.programming.kicks-ass.net>

On Tue, Sep 14, 2021 at 08:04:55PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 14, 2021 at 09:29:00AM -0700, Peter Oskolkov wrote:
> > In the version of the patchset that I'm preparing to send I've decided
> > to punt on the issue and just ask the userspace to deal with locking
> > the memory as it sees fit: mlock() is available and as far as I can
> 
> Sadly mlock() does not imply no faults. Someone had a too literal
> reading of the POSIX-RT spec (of which mlock is part) and figured that
> all that was required was to keep the page in memory, not avoid faults.
> 
> Linux has had this bahviour for ages, PREEMPT_RT has tried to change
> this, but so far to no avail. At some point sys_mpin() was proposed to
> meet the original POSIX-RT intent, but afaict that never actually
> happened.
> 
> In short, mlock() does not avoid minor faults, or even migration faults,
> which can take a fair while to resolve.

Also, even if it did, that would still not be acceptible because
userspace could fail to call mlock() at which point mis-behaving
userspace can deadlock the kernel, which is a no-no.

  reply	other threads:[~2021-09-14 18:16 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-08 18:49 [PATCH 0/4 v0.5] sched/umcg: RFC UMCG patchset Peter Oskolkov
2021-09-08 18:49 ` [PATCH 1/4 v0.5] sched/umcg: add WF_CURRENT_CPU and externise ttwu Peter Oskolkov
2021-09-08 18:49 ` [PATCH 2/4 v0.5] sched/umcg: RFC: add userspace atomic helpers Peter Oskolkov
2021-09-08 23:38   ` Jann Horn
2021-09-09  1:16     ` Jann Horn
2021-09-09 19:06     ` Peter Oskolkov
2021-09-09 21:20       ` Jann Horn
2021-09-09 22:09         ` Peter Oskolkov
2021-09-09 23:13           ` Jann Horn
2021-09-14 16:52         ` Andy Lutomirski
2021-09-14 18:11           ` Peter Zijlstra
2021-09-14 18:40             ` Andy Lutomirski
2021-09-15 15:42               ` Peter Zijlstra
2021-09-15 16:50                 ` Andy Lutomirski
2021-09-15 19:10                   ` Peter Zijlstra
2021-09-14  8:07       ` Peter Zijlstra
2021-09-14 16:29         ` Peter Oskolkov
2021-09-14 18:04           ` Peter Zijlstra
2021-09-14 18:15             ` Peter Zijlstra [this message]
2021-09-14 18:29             ` Peter Oskolkov
2021-09-14 18:48               ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 3/4 v0.5] sched/umcg: RFC: implement UMCG syscalls Peter Oskolkov
2021-09-09  1:39   ` Jann Horn
2021-09-14 16:51     ` Peter Oskolkov
2021-09-08 18:49 ` [PATCH 4/4 v0.5] sched/umcg: add Documentation/userspace-api/umcg.rst Peter Oskolkov
2021-09-14 16:35   ` Tao Zhou
2021-09-14 16:57     ` Peter Oskolkov

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